Hi there, i have an error while click on save button.

"System.NullPointerException: Attempt to de-reference a null object
Error is in expression '{!upload}' in component <apex:commandButton> in page aqthc:booking_funnel_settings: Class.aqthc.FileUploadController.upload: line 31, column 1".


<apex:page controller="FileUploadController">
  <apex:sectionHeader title="Visualforce Example" subtitle="File Upload Example"/>
 
  <apex:form enctype="multipart/form-data">
    <apex:pageMessages />
    <apex:pageBlock title="Upload a File">
 
      <apex:pageBlockButtons >
        <apex:commandButton action="{!upload}" value="Save"/>
      </apex:pageBlockButtons>
 
      <apex:pageBlockSection showHeader="false" columns="2" id="block1">
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File Name" for="fileName"/>
          <apex:inputText value="{!document.name}" id="fileName"/>
        </apex:pageBlockSectionItem>
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File" for="file"/>
          <apex:inputFile value="{!document.body}" filename="{!document.name}" id="file"/>
        </apex:pageBlockSectionItem>
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Description" for="description"/>
          <apex:inputTextarea value="{!document.description}" id="description"/>
        </apex:pageBlockSectionItem>
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Keywords" for="keywords"/>
          <apex:inputText value="{!document.keywords}" id="keywords"/>
        </apex:pageBlockSectionItem>
        
        <form action="/action_page.php">
        Select your favorite color:
            <input style="margin-left: 1px;" type="color" name="favcolor" value="#ff0000"/>
            <input style="margin-left: 30px;" type="submit"/>
        </form>
 
      </apex:pageBlockSection>
 
    </apex:pageBlock>
  </apex:form>
</apex:page>
===============================================================
public with sharing class FileUploadController {
 public List<aqthc__Healthcare_Settings__c> objHSList {get;set;}
  public Document document {
    get {
      if (document == null)
        document = new Document();
      return document;
    }
    set;
  }
 
  public PageReference upload() {
 
    document.AuthorId = UserInfo.getUserId();
    document.FolderId = UserInfo.getUserId(); // put it in running user's folder
 
    try {
      insert document;
    } catch (DMLException e) {
      ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading file'));
      return null;
    } finally {
      document.body = null; // clears the viewstate
      document = new Document();
    }
    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'File uploaded successfully'));
    aqthc__Healthcare_Settings__c objHc = new aqthc__Healthcare_Settings__c();
        objHc.Name = document.name;
        //objHc.aqthc__Background_Color__c = document  ;
        //objHc.aqthc__Background_Image_URL__c = document.FolderId;
        objHSList.add(objHc);
       insert objHSList;
    return null;
  } 
}

Please help!
Thanks